The present invention relates to an optical component and to a device for use thereof in an optical device which is provided with an autofocus arrangement and which provides a dark field illumination.
The fundamental mode of operation of a laser autofocus arrangement is knwon from German Pat. No. 3,219,503. With such a device, if it were desired to provide a dark field illumination, it would be necessary to perform circular masking-out of the central region of an illumination beam, for example, by rotating in a customary central stop. However, in such a case the function of the laser autofocus is made ineffective, and moreover the measurement light is likewise masked out.
In order to overcome this disadvantage, it would accordingly be necessary to construct the dark field stop as an infrared filter (IR filter), which absorbs 100% of the visible light, but transmits everything beginning with a wavelength range situated in the near IR. However, such a measure alone would lead to an undesired focussing displacement, since a plane-parallel IR filter plate causes a change in intercept length in the beam path.